This invention relates to moisture separator reheaters having a scavenging steam vent condenser, and more particularly to flow control devices for regulating vent flow rates while passing a two-phase mixture therethrough at a predetermined flow rate.
A scavenging steam vent condenser was developed to alleviate tube failure problems which were the result of cyclic tube temperatures caused by alternate flooding and draining of the tubes of moisture separator reheaters. A better operational understanding of a moisture separator reheator equipped with a scavenging steam vent condenser can be obtained by referencing R. L. Coit's copending patent application assigned to the assignee of the present invention and whose serial number, filing date, and title are respectively Ser. No. 650,836, Jan. 21, 1976, and "Apparatus For Increasing Effective scavenging Steam Within A Heat Exchanger Which Condenses Vapor Inside Long Tubes". The introduction of the scavenging steam vent condensers in moisture separator reheaters provided two additional passes therethrough, but created a problem which was non-existent in previous two-pass moisture separator reheater designs. That problem was the conveying away in a controlled fashion of the vapor and condensate which exit from the fourth pass of the moisture separator reheater. Before scavenging steam vent condensers were designed, separation of condensate and vapor exiting from the second pass was feasible in that the tube side flow chamber was divided into an inlet and an outlet portion with the outlet portion providing a sufficiently large plenum for separating the condensate from the remaining vapor. However, by converting moisture separator reheaters to four-pass heat exchangers, the available plenum size within the chamber at the exit from the fourth pass was only marginally sufficient to accomplish separation between the vapor and liquid phases. Separation of the two phases was desirable to allow predictable regulation of the vapor's flow rate being vented to a lower pressure source. The separated liquid and vapor portions would then each be controllable by standard, single-phase flow regulating devices.
Phase separation of the fluid exiting the fourth pass necessitated the use of an additional separation tank, a vapor conduit, and a condensate conduit. The separation tank had to be located on the exterior of the moisture separator reheater because the available volume within the chamber was of insufficient size to permit phase separation. The additional condensate conduit was required because condensate exiting the fourth pass was at a lower pressure than the vapor and condensate exiting from the second pass. This pressure difference would have caused much of the second pass's exiting vapor and condensate to "short circuit" the third and fourth passes and thereby promote inefficient operation of those passes.
It was determined that the most reliable and economically feasible means for venting the fluid exiting from the fourth pass of the moisture separator reheater was a single conduit line passing from a fourth pass exit manifold to a lower pressure source. However, to prevent increases in heat rate as a result of excessive venting and to avoid temperature cycling of the tube wall which can result in failure of the tubes and their surrounding welds, a highly reliable and predictable means for controlling the fluid flow rate through the third and fourth passes was required. It was discovered that much investigation during the 1960's was devoted to flashing, critical flow of saturated water because of concern for a major accident in the coolant system of nuclear reactor power plants. In particular, Fauske's paper entitled "The Discharge of Saturated Water Through Tubes" was extensively referenced in the development of this invention.